Let'* start with checking the sunroof drain tubes, because they are easy to check.

Open the sunroof rearward, and you will notice a small drain hole in each forward corner of the sunroof tub. One at a time, slowly pour water into each of those drain tubes and observe where the water comes out. If the forward tubes are hooked up and not plugged the water will drain out in the area just behind each front wheel. The drain outlet is just inside the fenderliner at the rear of the wheel opening. Passenger side drain tube drains to the passenger side, and driver to the driver side.

Now, move the sunroof to the closed position, and open it in the vent position (rear pops up). Repeat the water test for each of the rear drain openings. This time the water should drain out to the road just forward of the rear wheels.

If you pour water into any one of the four drain holes and the water doesn't drain out to the road in the areas I mentioned, you found at least one of your leaks. We can discuss fixes if that is the problem.

Maybe this will help understand your frost problem

Windshield frosting on INSIDE? - Car Talk

Thanks for all the replies and helpful info, gents. Right now though I'm in Skim'n'Save mode, no time to digest, much less take any action. The sunroof info helps, the Michael and Roy door info is good, and so is the Cartalk page. I have to be away for a couple of days now, maybe Friday will allow enough time for more delving. I'll probably end up at a body shop.

I tested the sunroof drainage, seems fine. The was a guess that the leak was from the heater box. Seems not so, as it'* dry on the underside of the dash. Thank you, I was quoted 10-12 hours labor to remove/replace the whole dash to install a new one. Btw, aside from labor a new one at GM = $233, new at Advance = $61.

My mechanic who nixed the heater box leak idea then sent me to his friend the auto glass guy. That guy thought right away he knew, it was something about the cabin particulate air filter box, seen this a few times on GM cars. He took off the wipers and detached the rain-tray cowl and told me I was missing a filter box "cover", that allowed the leak. I will say, the carpet wet area fluid is slightly tacky/oily like diluted antifreeze would be. There'* a recent history on this filter. Over the winter when snow was getting into my door frames my mechanic looked around and for some reason opened the filter box to discover no filter inside. He put a new filter in. Maybe he did something wrong? I did find this on the web yesterday: "In 03 the cabin filter was dropped as a factory part. So those cars had people thinking there was a cabin filter in place when there really wasn't." - that could be confusing, eh?

So anyway, after the glass guy told me that about the "cover" I called a few parts places and no one had one to sell, seems like a junkyard item. So another call to the mechanic. He said bring the car in Monday with the specifics from the glass guy and he'd probably be able to get it delivered from one of the junkyards.

OK, plan set. But then I went out to look and see what I could see around this filter zone. The only "cover" I could see was there, intact, looking like a glove box cover but with 2 tabs instead of a handle. I opened it up to see a filter and what looks like the proper frame. Then, just scoping around, I noticed a suspect gap between the windshield glass and what I later found out is called the "wiper cowl" or "rain-tray" or "drip-tray". It turns out this is a common entry point for rain to soak one'* front carpets. Here'* a There are also a series of 'buttons' on the cowl, a couple of which are missing. I am forced to home park close to a tree that constantly drops oak-like seed flyers onto my windshield. They accumulate at the bottom and in the mesh areas of the cowl. They also get caught at the bottom of my side windows. Btw, the door leak procedure suggested is way beyond my old creaky painful back and skills. I'd probably end up with bare doors. So, I'm frequently found picking out this crud. Now, I don't know if this gap was there before the glass guy moved the cowl or not, and I don't know if the empty button spaces could be a leak source. I have to go back to the glass guy Monday to talk about all this, including the doors, before I go to the mechanic to order junkyard parts.

Maybe it'* a mix of mostly rain with a little antifreeze? Meanwhile, last night I put a big towel down at the bottom of the windshield, got soaked overnight...

UPDATE: I went back to the glass guy this morning and they don't have a clue what might be missing in the air filter box, only that "something Must be". Hmm, so then over to the mechanic, "no time to look today, come back tomorrow". OK. Later, I went to a much more highly regarded body shop, out of town, just for a "free diagnosis", and the shop boss immediately zeroed in on the sunroof drainage. I told him I'd already checked the drain on the passenger side and it worked fine. Well, I'd only put about 3oz of water down, not exactly a prolonged rain or downpour. He said it'* all one drain system, you have to check both sides, "a clog on one side will cause an overflow on the other side". So checking the driver side roof drain. It didn't drain out that side of the car. So then he looked in behind the driver front wheel well, behind the rubber splash guard. It was full of debris, mostly tree stuff. He dug at it and used a strong blower to clean it out. He then checked behind the passenger-side rubber and there was some crud but not nearly as much. After that he had me close up all the glass and he hosed down the whole zone, seeing lots of good drainage. He then pronounced he thought the problem was found and fixed, No Charge! We'll see, as my brother Will C. was fond of saying.

p.*. since I was there anyway, I asked him about fixing the many body scuffs that had show up in the last few months. I've had the car 5 years with NO scuffs and I hadn't scraped onto anything this year or Ever. He said the car might have been cosmetically dolled up just before I bought it and it'* taken 5 years for the makeup to fade!

Thanks for updating us, sounds like it may be fixed, hope it is!

Mine was a simple fix. The battery vent tube grommet was not sealed to the floor pan sheet metal. Pushed the grommet back thru and the inside fogging/frost disappeared. Amazing how little an opening would cause all that moisture inside.

It wasn't the sunroof drain tubes. Months later my front passenger carpet was still soaking. I'd pretty much given up on a fix. NOW I think the wet carpet, and hopefully the inside frost, is all due to a clogged AC condensate drain tube, which I got unclogged at my local shop yesterday. It gets clogged up with the same tree crud that clogged my sunroof drains. A pretty common problem, here'* a good link - GM A/C Condensate Rubber Elbow Drain Pipe Unclogging
GM A/C Condensate Rubber Elbow Drain Pipe Unclogging & Cleaning Guide - 1997 To 2003 Pontiac Grand Prix - Passenger Floorboard Water Leak Problem If you look around, there are other info sites on this, even Youtube videos, but that link is the best I found.

I'm no mechanic or Sherlock but I arrived at a guess that the wet carpet was AC related because of a few factors:

1. my AC wasn't working well, so I stopped using it for a few weeks despite it being hot out
2. I noticed that the odd wet spots in the driveway under my tail pipes disappeared
3. the carpet dried out at the same time!
4. at that point my old noggin circled around to the AC not running, and I started doing some net research

As that site says, "Often the problem can be resolved easily by removing the rubber elbow attached to the end of the HVAC system'* condensate drain pipe and cleaning out any obstructions." Well, the shop could only get at my pipe with difficulty up on a lift, so they blew air through it from the motor side. Curiously, to my mind, GM designed this drain tube arrangement so that one tube end goes through the firewall into the area just above that footwell. I don't get it GM, why would condensate need to drain into a footwell? You can see some photos here:
AC drain tube blocked : 2000-2005 (other than GXP)
Well they probably had a reason but I don't need to know.

As for the inside frost issue, last winter, due to some reading I'd done somewhere, for the first time ever I would use the AC to defrost the OUTside of the windshield. So maybe this condenser drain tube issue created a carpet just wet enough (underneath) to make the air moist enough to frost. I won't know til winter.

Meanwhile, after the unclogging, I had them test the AC. It was 49F on the passenger side - nICE cold - but barely cool on the driver side. After fiddling with the dual climate controls, setting them both to the lowest, no change in symptoms. I then read that it'* probably the actuator for the driver side air - "Since you're not getting any cold air on the driver'* side, it appears that the left door is stuck in the non-cooling position. You may want to try resetting the system by pulling the fuse for the A/C system (or disconnecting the battery) since the unit performs a startup routine that moves all actuators to their extreme positions to calibrate them. If that doesn't work, then the actuator is dead and needs to be replaced. Not a major problem."

Now I've got to try to track down the fuse, or maybe last resort disconnect the battery...

A low refrigerant charge can give the same symtoms ... cold out the driver side and not so much out the passenger side.

FIXED! Just got back from over 2 hrs in the shop, all labor, no parts or even freon. Removing the HVAC fuse did nothing. Same no help from disconnecting the main car battery. Then he visually checked the 'mode and temp servos' and 'checked the temp sensors and control head' and there was 'NO charge for the driver side voltage of control ****". Btw, this is all over my head, my eyes just glaze over. Then he found that the overall system was throwing 7 codes. I have them written down if anyone cares. He then did whatever they do to 'clear the codes', and then the cold air came back on the driver side. As a final move, he checked the freon pressure, thinking it might be low, but no it was 2 pounds high so he adjusted that.

This was at Meineke, and this mech was I'd guess over 60, had been in it all his life, was trained by both GM and Honda, had had his own shops, and seemed extremely knowledgeable. Then, even though Meineke gets $85 an hour labor and the work took over 2 hrs (and I was there watching the whole time), they charged me $111.